This Thursday, April 25, 2013 photo shows a 1941 farewell letter from a Butrimonys Jew in hiding at the United States Holocaust Memorial Museum during a preview of the new exhibit "Some Were Neighbors: Collaboration & Complicity in the Holocaust" in Washington. The exhibition, opening April 30, 2013, includes interviews with perpetrators of collaboration and complicity in the Nazi genocide. (AP Photo/Carolyn Kaster)

In this Thursday, April 25, 2013 photo, United States Holocaust Memorial Museum curator Susan Bachrach points out a 1942 photo of Ukrainian auxiliary police killing Jewish women under the supervision of German army personnel and in the presence of Hungarian troops during a preview of the new exhibit "Some Were Neighbors: Collaboration & Complicity in the Holocaust" in Washington. The exhibition, opening April 30, 2013, includes interviews with perpetrators of collaboration and complicity in the Nazi genocide. (AP Photo/Carolyn Kaster)

WASHINGTON (AP) — Elderly Holocaust survivors and the veterans who helped liberate them gathered for what could be their last big reunion Monday at the U.S. Holocaust Memorial Museum.

Nearly 1,000 survivors and World War II vets joined with former President Bill Clinton and Nobel Peace Prize laureate and Holocaust activist Elie Wiesel to mark the museum's 20th anniversary. Organizers chose not to wait for the 25th milestone because many survivors and vets may not be alive in another five or 10 years.

"We felt it was important, while that generation is still with us in fairly substantial numbers, to bring them together," said Museum Director Sara Bloomfield.

Washington has many monuments and memorials that offer something special for visitors from around the world, Clinton told the crowd, "but the Holocaust memorial will be our conscience."

Since the museum opened, the world has made huge scientific discoveries, including the sequencing of the human genome, Clinton said.

"Every non age-related difference you can see in this room and across the globe, every single one is contained in one half of 1 percent of our genetic makeup ... but every one of us spends too much time on that half a percent," Clinton said. "That makes us vulnerable to the fever and the sickness that the Nazis gave to the Germans.

"And that sickness is very alive all across the world today."

The occasion marked a reunion of sorts for Clinton and Wiesel: Both were on hand to dedicate the museum at its 1993 opening. On Sunday night, the museum presented its highest honor to World War II veterans who helped end the Holocaust. Susan Eisenhower accepted the award on behalf of her grandfather, U.S. Gen. Dwight D. Eisenhower, and all veterans of the era.

The federally funded museum also launched a campaign to raise $540 million by 2018 to keep the memory of the Holocaust alive and to combat anti-Semitism, Holocaust denial and contemporary genocide.

It has already secured gifts totaling $258.7 million in its quest to double the size of the museum's endowment by its 25th anniversary. Also, a $15 million gift from Holocaust survivors David and Fela Shapell will help build a new collections and conservation center.

Bloomfield said organizers wanted to show Holocaust survivors, veterans and rescuers that the effort will continue to honor the memory of 6 million murdered Jews, in part by working to prevent genocide in the future. For instance, a study released by the museum last month found the longer the Syrian conflict continues, the greater the danger that mass sectarian violence results in genocide.

The museum's theme for its 20th anniversary is "Never Again: What You Do Matters."

Vera Greenwood, who was born in Berlin and remembers seeing Hitler with Nazis marching in the street, said her father knew they had to leave when he was forced out of his job as a lawyer. She remembers Nazi officers coming to their house and taking her father's books.

"Though I was very young, I knew something was very wrong," said Greenwood, now 84. "I still feel we were very lucky to survive."

Her family moved to Palestine with a British visa and ended up fighting for Israel's independence. Greenwood lived in Israel for 30 years before immigrating to the U.S.

She and her husband, Fred, who survived the Holocaust in Holland as a child by being hidden and passed from house to house, wanted to be part of the last large reunion of survivors.

"In 10 more years, most of us will be gone," Greenwood said, noting the museum is a way to keep their stories alive.

Herman Zeitchik, 89, of Silver Spring, Md., was a young U.S. Army soldier when his unit landed at Normandy in the mission to liberate Europe. He remembers coming across the Dachau concentration camp unexpectedly in southern Germany.

"They never told us there was a concentration camp, but we smelled it," Zeitchik said. "We smelled the burning flesh."

Later during a patrol, Zeitchik saw people held within the camp's chain-link fence. "That's the first time I knew about the concentration camps," he said.

Dachau was the first regular Nazi concentration camp, established in 1933. The Americans liberated Dachau's prisoners in 1945.

The museum continues collecting objects, photographs and other evidence of the Holocaust from survivors, veterans and archives located as far away as China and Argentina. Curators expect the collection to double in size over the next decade.

This week, the museum is opening a special, long-term exhibit titled "Some Were Neighbors: Collaboration and Complicity During the Holocaust." It includes interviews with perpetrators never shown before.

Curator Susan Bachrach said the exhibit's research challenges the idea that the Holocaust was primarily about Hitler and Nazi leaders. Surveys at the museum show that's what most visitors believe.

"That's very comforting to people, because it puts distance between the visitors and who was involved," Bachrach said.

So, the museum set out to look at ordinary people complicit in the killing and persecution of millions of Jews through greed, peer pressure, a desire for career advancement or other factors beyond hatred or anti-Semitism. The exhibit includes images of bystanders looking on as Jews were led away.

Focusing only on fanatical Nazis would be a serious misunderstanding of the Holocaust, Bloomfield said.

"The Holocaust wouldn't have been possible, first of all, without enormous indifference throughout Germany and German-occupied Europe, but also thousands of people who were, say, just doing their jobs," she said, such as a tax official who collected special taxes levied against Jews.

Much more is still being learned about the Holocaust, Bloomfield noted.

The museum is compiling an encyclopedia of all incarceration sites throughout Europe. When the project began, scholars expected to list 10,000 such sites. Now the number stands at 42,000.

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